The present invention relates to a semiconductor device and a method for fabricating the semiconductor device, more specifically a semiconductor device having a MOS structure and a method for fabricating the semiconductor device.
As an insulation film for MOS (Metal Oxide Semiconductor) structure, SiO2 film has been conventionally predominantly used. SiO2 film has been increasingly thinned as semiconductor devices are increasingly micornized. However, it is said that the limit of thinning SiO2 film is about 3 nm. Then, studies of forming the gate insulation film of materials of higher dielectric constants than that of SiO2 film are being made.
Al2O3 is noted as a material which well matches with the conventional semiconductor processes and additionally has a higher dielectric constant than SiO2 film.
A proposed semiconductor device which uses Al2O3 as a material of the gate insulation film will be explained with reference to FIG. 13. FIG. 13 is a sectional view of the proposed semiconductor device.
As shown in FIG. 13, an element isolation region 110 for defining en element region 108 is formed on a silicon substrate 106. A gate insulation film 114 of Al2O3 is formed on the silicon substrate 106 in the element region 108. A gate electrode 116 of polysilicon is formed on the gate insulation film 114. A cap film 118 is formed on the gate electrode 116. Dopant diffused regions 120a forming shallow regions of an extension source drain are formed on silicon substrate 106 on both sides of the gate electrode 116. A sidewall insulation film 122 is formed on the side walls of the gate electrode 116. Dopant diffused regions 120bforming deep regions of the extension source drain are formed on both sides of the gate electrode 116 with the sidewall insulation film formed on the side walls. The dopant diffused regions 120a and the dopant diffused regions 120b form a source/drain diffused layer 120 of the extension source drain structure. Thus, the proposed semiconductor device is constituted.
However, when Al2O3 is used as a material of the gate insulation film 114, the flat band voltage largely shifts. FIG. 14 is a graph of the C-V characteristics. Gate biases are taken on the horizontal axis. Capacitances are taken on the vertical axis. As evident in FIG. 14, when Al2O3 is used as a material of the gate insulation film, the flat band voltage shifts by about 0.4 V in comparison with that in the case that SiO2 is used as a material of the gate insulation film.
Furthermore, the flat band voltage shift tends to be larger as the film thickness of the Al2O3 film is thinner. FIG. 15 is a graph of relationships between the film thickness of the Al2O3 film and the flat band voltage shift. Oxide film converted film thicknesses are taken on the horizontal axis, and shifts of the flat band voltage are taken on the vertical axis. The measuring conditions are as follows. That is, a p type silicon substrate was used as the substrate, nitrogen added Al2O3 film was used as the insulation film, and polysilicon film was used as the electrode. The measuring temperature was −25° C. As evident in FIG. 15, the flat band voltage shift tends to be higher as the Al2O3 film is thinner.
When a flat band voltage shift is large, a required threshold voltage cannot be obtained. Accordingly, even in a case that Al2O3 film, etc. are used as a material of the gate insulation film, techniques which can suppress the flat band voltage shift have been expected.